1. Field of the Invention
The invention concerns a measuring cable travel sensor and more particularly a measuring cable travel sensor having a longitudinal drive for displacement of the cable drum.
2. Background of the Invention
Measuring cable travel sensors occur in many different configurations for example for determining the precise position of a given component which in particular can move over considerable travel distances, for example the cabin of an elevator. A typical measuring cable travel sensor of that kind and for that purpose has a pull element such as a measuring cable which is wound on a cable drum which is biassed in the cable winding-on direction. The free end of the measuring cable is connected to that component whose position is to be determined, for example as mentioned above an elevator cabin. The prestressing force for urging the cable drum in the winding-on direction is produced for example by way of a flat spiral spring which for example is arranged coaxially with respect to the cable drum and is non-rotatably connected thereto.
The cable drum is also typically coupled to a detection unit for recording the revolutions or angular distances through which the cable drum passes in the winding-on or unwinding direction, and which in addition by way of a suitable electronic evaluation system determines the length of the pull element or measuring cable which has been drawn off the cable drum.
In order to simplify that operation of determining the drawn-off length of the measuring cable and to ensure an accurate measurement result, the measuring cable is wound in only a single layer on the periphery of the cable drum, with the turns of the wound cable on the cable drum being disposed in axially juxtaposed relationship. As a result, one revolution of the cable drum will always correspond to exactly the same length of the pull element or measuring cable.
In order to ensure that the measuring cable is wound on to the drum in only a single layer and in order to prevent the measuring cable from jumping over on to the first layer to start to form a second layer there over when winding the measuring cable on to the cable drum, it is generally necessary to take suitable mechanical steps involving appropriate structure on the travel sensor to obviate this occurrence.
In that respect there is an additional difficulty insofar as such measuring cable travel sensors often have to be used in an adverse environment, for example involving a high level of fouling and contamination. For that reason the measuring cable travel sensor has to be disposed in a housing which affords sealing integrity, and the measuring cable which is passed out of the housing also has to be taken out of the housing by way of a cable guide passing through the wall of the housing, which also offers the greatest possible level of sealing integrity in relation to the cable.
In order to ensure that the measuring cable is wound on the cable drum in a neat and tidy fashion in a single layer, one possible way of achieving that is for the cable drum to be very short in its axial direction. If in addition the cable entrance is at a sufficient distance from the cable drum, then the measuring cable, on moving from the cable entrance to the cable drum, runs only at such a slight degree of angular deflection as to still be acceptable, and such an arrangement automatically causes the cable drum which is biassed in the winding-on direction to have the measuring cable wrap there around in only a single layer.
Even if the diameter of the cable drum is selected to be relatively large to provide a compensatory effect, that nonetheless entails a limitation in terms of the maximum possible length of the measuring cable. In addition, that affords a structural configuration which is very large in the radial direction, and a single revolution of the cable drum corresponds in that case to a relatively long peripheral length and thus a relatively long length of cable, so that the level of resolution of the measuring cable travel sensor is restricted in consequence.
If the above-indicated limitations give rise to the need for the cable drum to be of a substantially greater axial extent, then the cable entrance must be moved relative to the cable drum when the measuring cable is being wound on to or unwound from the cable drum, so that the cable entrance is disposed always substantially at the radial plane at which the measuring cable is being wound on to or unwound from the cable drum at that time. In that respect, and for that purpose, while it is possible for the cable entrance to be displaced relative to the cable drum which is arranged in axially fixed relationship in the housing, it is also possible to adopt the reverse arrangement.
At first view, it may admittedly be an easier course of action for the relatively small and light cable entrance to be adapted to be axially displaceable, rather than the rotatably mounted cable drum, but from the point of view of affording good sealing integrity for the measuring cable travel sensor with respect to its ambient conditions, a preferred arrangement is for cable guide means to be arranged fixedly on the housing, so that the measuring cable drum is axially displaceable relative thereto.
In that respect attention may be directed for example to U.S. Pat. No. 4,443,888 in which the measuring cable drum is rotatably mounted directly on a central screwthreaded spindle. The pitch of the screwthread on the spindle establishes the relationship between rotary movement and longitudinal displacement of the cable drum. The rotary movement of the cable drum is transmitted by way of an eccentric entrainment means to the return unit of the cable drum for urging it on the winding-on direction.
Reference may also be made to European patent No. 0 778 239 disclosing a screwthreaded spindle of a hollow configuration, wherein disposed in the interior of the hollow screwthreaded spindle in coaxial relationship therewith is a shaft, for example a splined shaft, on which the cable drum is arranged longitudinally displaceably but non-rotatably by way of one of its end plate portions, while the other end plate portion of the cable drum is in screwthreaded engagement with the male screwthread on the screwthreaded spindle. The longitudinal drive means in the form of the screwthreaded spindle on the one hand and transmission of the rotary movement of the cable drum to the rotary angle sensor and/or the return drive by means of the splined shaft on the other hand are thus disposed in the center of the cable.drum in mutually coaxial relationship.
A disadvantage with those structures however is that the degree of longitudinal displacement depends on the pitch of the screwthread on the screwthreaded spindle and the spindle nut co-operating therewith. If for example the travel sensor uses a measuring cable of relatively great thickness, for example because the tensile loading acting on the measuring cable is relatively high, then when the cable drum rotates, a greater degree of longitudinal displacement thereof is required than when using a thinner measuring cable.
In the above-mentioned conventional structures, in order to solve this problem the screwthreaded spindle and the spindle nut co-operating therewith must to be, exchanged, which entails complete dismantling of the measuring cable sensor, including fitting and thus, in the fitting operation the return device which in general is a flat shaped spring in the form of a spiral spring, must be again prestressed. This can scarcely be effected on site even by a specialized fitter, quite apart from the large amount of time that this necessarily entails.
An object of the present invention is to provide a measuring cable travel sensor adapted to provide for longitudinal displacement of the cable drum within its housing using a simple structure, wherein the relationship between rotary movement of the cable drum and longitudinal displacement thereof can be repeated and easily altered.
Another object of the present invention is to provide a measuring cable travel sensor with a longitudinally displaceable drum for adaptation to the position at which the measuring cable passes on to and off from, which affords in particular a very low-friction support configuration for the cable drum combined with a simple and inexpensive design configuration and easy travel sensor maintenance options.
Still another object of the present invention is to provide a measuring cable travel sensor with a longitudinally displaceable drum, which affords greater ease of and reliability in operation, enhanced measurement accuracy and simplicity of structure.
In accordance with the present invention the foregoing and other objects are attained by a measuring cable travel sensor comprising a housing and a cable drum which is arranged in the housing rotatably and longitudinally displaceably relative to the axis of rotation of the drum in the longitudinal direction thereof. The travel sensor further has a cable guide means for guiding the measuring cable, being arranged in fixed relationship with the housing. The travel sensor further includes a longitudinal drive means for displacement of the cable drum, comprising a spindle drive means including a screwthreaded spindle which is screwable relative to a spindle nut thereon. The longitudinal drive means has an additional transmission means for transmitting rotary movement of the cable drum to the spindle drive means.
Further in accordance with the principles of the present invention the foregoing and other objects of the invention are attained in a second aspect by a measuring cable travel sensor including a measuring cable and a cable drum having a winding cylinder with an external winding surface, for the measuring cable to be wound on to the external winding surface of the cable drum. The cable drum is mounted in a housing. The travel sensor further includes at least one retardation or braking magnet which is so arranged at a point that is non-rotatable with the cable drum that it has a magnetic retardation or braking action in contact-less manner in opposite relationship to the direction of winding-on rotation of the cable drum on the cable drum at an eccentric region of action thereon. At least in the region of action the cable drum includes electrically conductive material.
In accordance with the principles of the present invention the foregoing and other objects of the invention are attained in a third aspect by a measuring cable travel sensor including a measuring cable and a cable drum having a winding cylinder with an external winding surface, for the measuring cable to be wound on to the external winding surface of the cable drum. The cable drum is mounted in a housing. At least one retardation or braking magnet is eccentrically arranged on the cable drum in such a way that it has a retardation or braking effect magnetically in contact-less manner on the housing in the direction of rotation. The housing is of electrically conductive material at least in the region of action of the magnet.
In accordance with the principles of the present invention the foregoing and other objects of the invention are attained in a fourth aspect by a measuring cable travel sensor including a measuring cable and a cable drum having a winding cylinder with an external winding surface, for the measuring cable to be wound on to the external winding surface of the cable drum. The cable drum is mounted in a housing. The sensor further includes at least one holding magnet arranged at the cable drum radially within the external peripheral surface of the winding cylinder of the cable drum, for holding the measuring cable radially inwardly against said winding surface, wherein the measuring cable comprises magnetisable material.
In accordance with the principles of the present invention the foregoing and other objects of the invention are attained in a fifth aspect by a measuring cable travel sensor including a measuring cable and a cable drum having a winding cylinder with an external winding surface, for the measuring cable to be wound on to the external winding surface of the cable drum. The cable drum is mounted in a housing. The sensor further includes a sliding band, for example a band of textile material, more particularly a sliding band or band of other slidable, non-abrasive material, which is passed around at least a part of the external periphery of the external winding surface of the cable drum, which external winding surface is partially or entirely movable with the measuring cable, the band being passed around said winding surface at a small spacing and/or in dragging or frictional relationship at the external periphery of the winding with the measuring cable, while the spacing or the contact pressure of the band against the measuring cable winding is adjustable.
As will be noted from the description hereinafter of preferred embodiments of the present invention, in the first aspect thereof, the fact that the longitudinal drive for displacement of the cable drum along the direction of its axis of rotation not only comprises the spindle drive means but an additional transmission means that the additional transmission can be of such an arrangement and configuration that it is easily possible to make a change in the transmission effect and ratio of the additional transmission means. If the additional transmission means comprises for example a belt transmission such as a toothed belt transmission then the belt pulleys can be replaced by pulleys of a larger and/or smaller diameter as appropriate. If the transmission means comprises a gear transmission arrangement, then the relevant gears can be appropriately changed.
By virtue of the provision of a belt tensioner, in that case it is not even necessary for the belt such as a toothed belt also to be changed upon a change in the belt pulleys. Such an advantageous arrangement, which is readily accessible, is afforded if the screwthreaded spindle and the cable drum rotate about mutually parallel axes, and in that respect in particular are supported in end plate portions of the housing. In that respect preferably the shaft which also rotates with the cable drum is in the form of a central shaft and the cable drum is carried thereon non-rotatably but longitudinally displaceably, directly or indirectly.
Thus for example the cable drum could be rotatably supported in a carriage or slider which is non-rotatably arranged longitudinally displaceably on the central shaft.
Another option in this respect provides that the cable drum is admittedly longitudinally displaceably and non-rotatably mounted on the central shaft directly or by means of an intermediate portion which also rotates therewith, but it is nonetheless guided in the axial direction in a slider or carriage. The axial movement then only has to be transmitted to that slider or carriage, and not directly to the cable drum.
That affords in particular the advantage that the axial mounting arrangement for supporting the cable drum in the slider or carriage can be of a particularly low-friction configuration, for example by involving a contact-free mounting by means of magnets, more especially maintenance-free permanent magnets.
For that purpose, arranged in the end plate portions of the cable drum on the one hand and the oppositely directed surfaces of the slider or carriage on the other hand are magnets which are oriented axially, in respect of their polarity, for example annular magnets, best in coaxial relationship with the axis of rotation of the cable drum, being directed in opposite relationship with the same poles. As therefore at the two ends the magnets of the cable drum are repelled by the magnets of the carriage or slider, the cable drum is always automatically set to a central position and in contact-free relationship on both sides with respect to the carriage or slider in the axial direction.
The option of easily changing the transmission ratio affords in particular the possibility of so selecting the longitudinal displacement of the cable drum that the individual turns or windings of the measuring cable on the cable drum are applied to the cable drum in closely juxtaposed relationship but without making contact with each other, in such a way as to be adapted to the respective cable diameter, which can contribute to reducing the wear that the cable suffers on being wound on to and unwound from the cable drum.
Preferably besides the cable drum the longitudinal drive means, that is to say the spindle drive means and the additional transmission means, are accommodated in sealed relationship within the housing and more specifically in particular in the same internal space in the housing, and are thus protected from fouling and contamination from the ambient atmosphere. The central shaft and also the screwthreaded spindle are preferably supported in the end plate portions of the housing, while the housing preferably comprises one or more longitudinally extending shaped members, for example extrusion portions, which are closed by end plate portions to form the closed housing structure.
The end plate portions are screwed on to the shaped member or members at the ends thereof in a simple fashion by virtue of the shaped member or members being provided, in the internal surfaces thereof, with screw passages in the form of grooves which are open towards the inside of the respective shaped portion and which afford an undercut configuration and which are of an approximately round cross-section. Accordingly, self-tapping screws can be screwed into the ends of the screw passages of that round cross-section.
The return drive for the cable drum which is generally a flat spring in the form of a spiral spring or a plurality of flat springs of that kind which are arranged in axial succession, in the form of a whole flat spring cascade, are preferably arranged coaxially and more particularly directly on the central shaft, preferably outside the housing for the cable drum, in a housing cover portion which is to be separately fitted to the main housing. The same consideration applies, preferably in regard to the opposite end, to the central shaft for the rotary angle sensor for detecting the rotary and angular movements of the cable drum within the housing.
So that the measuring cable can traverse the entire axial length of the cable drum if required, the central shaft, in the interior of the housing, must enjoy an axial length of approximately double the axial extent of the cable drum.
When the cable drum is supported in a slider or sliding carriage, the bearings for the cable drum are disposed at a mutual spacing corresponding to the axial extent of the cable drum, while when the cable drum is screwed directly along a screwthreaded spindle, only the screwthreaded spindle can be supported, more specifically with a distance between its mountings which approximately corresponds to double the axial extent of the cable drum, in the end walls of the housing.
The cable entrance member which is arranged fixedly in the housing, by virtue of the displaceability of the cable drum, can readily be disposed axially in the center of the housing. The cable entrance member substantially comprises a generally cup-shaped main body with a through bore in the bottom thereof, the measuring cable passing centrally through the main body and the bore therein.
Arranged in the main body, in succession from the outside inwardly in relation to the housing in which the main body is fitted, are a damping element on the outward side for damping impingement there against of a connecting portion carried on the free end of the measuring cable, then a cable guide means of a material which is highly resistant to wear and thereafter at an axial spacing preferably a plurality of cable scraper members. The cable scraper members are preferably of a plate-like configuration and bear in close contact against the outside surface of the cable. The cable guide means is of a relatively large axial extent and has an opening which enlarges in a trumpet-like configuration from the inside outwardly.
Packings of grease or other material which have a sealing action and which absorb dirt and other contamination or fouling to prevent it from passing into the housing can be arranged between the individual scraper members. Arranged at the inwardly facing orifice of the main body is a further cable guide means which enlarges in a trumpet-like configuration towards the interior of the housing of the travel sensor.
In regard to the further aspects of the invention as outlined above, involving a retardation or braking magnet on the housing or the cable drum, as will further become apparent from the description hereinafter of preferred embodiments of the invention, it is possible to limit the speed of rotary movement of the cable drum when the measuring cable is winding there onto by the use of the contact-less magnetic braking assembly. The components involved in that structure are to comprise an electrically conductive material.
The braking moment can be generated between an eccentrically disposed region of the cable drum, preferably a region which is in the proximity of the external periphery thereof, and a stationary point which is thus non-rotatable with respect to the cable drum, for example a part of the housing of the cable drum.
As the measuring cable is wound on to the cable drum on the radially outwardly disposed peripheral surface thereof, the magnets are preferably oriented in a longitudinal direction, that is to say parallel to the axis of rotation of the cable drum, between two components which are adjacent to each other in that axial direction.
In such an arrangement, by virtue of the rotary movement of the cable drum, and irrespective of the spacing of the retardation or braking magnet relative to the component to be influenced thereby, an eddy current is firstly produced in the component which carries the retardation or braking magnet. The consequence of that eddy current is a magnetic field which is closed by way of the portion which is not in the magnetic field, thereby producing a braking moment.
Besides the strength of the magnet used, the braking action is determined to a very great extent by the spacing between the magnet and the component to be influenced thereby, and it is for that reason that this spacing should preferably be adjustable.
The respective component which is to be subjected to the influence of the braking magnet must comprise electrically conductive material, for example aluminum. When the magnet or magnets are arranged on the cable drum, that increases the inertial mass thereof, and that therefore affords the further aspect of the invention which entails arranging the braking magnet or magnets at a stationary point for example on the housing. That would also remove the compulsion for the provision of at least a pair of braking magnets on the cable drum, as a single magnet results in a cable drum unbalance, and that arrangement also makes available more space for example for fitting a magnet holder for adjustability of the air gap.
A consideration which is in favour of arranging the magnet or magnets on the cable drum however is the possibility that the magnet or magnets can be used at the same time as a holding magnet or magnets, thus providing a functionally combined unit.
In this respect, in this specification the term holding magnet is used to denote a magnet which is intended to prevent the measuring cable from climbing up or rising away from the winding surface formed by the external surface of the winding cylinder portion of the cable drum, insofar as the material of the measuring cable, which for this purpose is necessarily magnetisable, is drawn radially inwardly by magnetic force towards that winding surface. Accordingly holding magnets of that kind have to be disposed radially within the winding surface on the cable drum, and preferably therefore fixed on the inside of the winding cylinder which consists of a thin material, preferably in turn distributed over the periphery of the cable drum. In this case also the recommendation is for the axis of magnetisation of the magnet or magnets to be oriented parallel to the axis of rotation of the cable drum, that is to say, it is recommended to provide a bar magnet, in particular a permanent magnet, which extends in the longitudinal direction of the cable drum.
In both cases the magnetic force can be increased by the provision of pole shoes or pole pieces, that is to say involving close contact of iron materials against at least one outside of the magnet, in order thereby to reduce the level of magnetic losses. Preferably, the arrangement does not involve any covering by a pole piece in that direction in which the field lines are required to pass into or out of the magnet freely. Therefore, primarily cup-shaped pole pieces are preferred, which in the case of the retardation or braking magnet are directed with their open side towards the component to be influenced by the magnetic effect and which, in the case of the holding magnet, are directed with their open side radially outwardly towards the measuring cable.
It is also possible to achieve an increase in the level of retardation or braking force by using rare earth magnets, that is to say magnets with components of samarium, cobalt, neodymium and/or boron. In particular rare earth magnets of that kind can be used to produce disk-shaped magnets whose magnetisation axis extends in parallel relationship to the thickness of the disk through the disk and/or which in that case can be magnetised differently in a sector-like configuration.
Disk-shaped magnets of that kind can be disposed in the constricted conditions in terms of space of measuring cable travel sensors, more easily than elongate bar magnets.
In particular such disk-shaped magnets can be accommodated in the end of screwthreaded pins or bolts as magnet holders which, in the component carrying them, can be moved by screwing closer towards or further away from the component to be influenced by the magnetic effect, in order thereby to provide for adjustability of the desired effect.
As an alternative and/or supplemental to the holding magnet or magnets, as indicated above, it is further possible in accordance with the invention to pass around the outside periphery of the movable cable drum a sliding band or belt, in particular a textile band or belt or a felt band or belt or more particularly a belt of plastic material such as PE, POM or PTFE, which when the measuring cable is wound correctly on the cable drum does not involve any contact with the wound turns of the measuring cable or bears against them without applying any force thereto, but which in contrast applies a force to the measuring cable if it climbs up on to a previous turn already formed on the cable drum.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of preferred embodiments of the invention.